1. Field of the Invention
The present invention relates generally to an apparatus and method for generating binaural beats for brainwave induction, and, in particular, to an apparatus and method for generating binaural beats for brainwave induction from a stereo audio signal.
2. Description of the Related Art
Generally, a brainwave inducer refers to a device for permitting both ears to hear sine waves having a predetermined frequency difference and simultaneously generating lights having the same frequency difference to both eyes of the user, to induce generation of a brainwave having a desired frequency. Such a brainwave inducer has been commercialized and popularized.
According to brainwave research based on the nerve current, which is a field of neuroscience involving study of the brain, it is known that when an external stimulus exerts influence on the brain, transition of brainwave is possible. For example, if a person in his/her beta phase is stimulated by a wave of 10 Hz, the person falls in a stabilized state where he/she is relaxed. The simplest approach to provide such external stimulus is an acoustic method, and a so-called binaural beat technique is used.
The ‘binaural beat’ refers to a sound which is generated by modulating an input audio signal with a sound in a human's audible frequency band (20˜20,000 Hz) and effectively mixing the modulated sounds to influence the brain, in order to form an external wave affecting the brain since the humans cannot perceive low frequencies. That is, an 11 Hz frequency belonging to the alpha phase is an inaudible frequency band, and a frequency difference of 11 Hz is given to the brain by providing a 300 Hz audible frequency band to the left ear and a 311 Hz sound to the right ear using a stereo sound. In this case, the two sounds are mixed in the brain, so the brain receives the frequency difference of 11 Hz, undergoing stimulation by brainwave induction.
For 300 Hz−311 Hz, when 300 Hz is provided to the left ear and 311 Hz to the right ear, the brain perceives 300 Hz−311 Hz=11 Hz.
A human brainwave is an electric signal occurring within the head, and its origin is a current generated by ions that pass through cell walls of the excited neurons. The brainwave is measured through (+, −) electrodes attached on the human's head surface using, for example, Electro Encephalo Graphy (EEG) equipment, and a voltage detected from the (+, −) electrodes should be amplified approximately 6 million times, so the brainwave can be measured, making it possible to indirectly comprehend a state of the brain. The measured brainwave represents a different mental state of the person according to its frequency range, and a relationship between the brainwave and the mental state with respect to the frequency range is shown in Table 1.
TABLE 1Frequency rangeBrainwaveMental state>40HzGamma wave13~40HzBeta waveNervous7~13HzAlpha waveRelax4~7HzTheta waveSleep, meditation<4HzDelta waveVery deep sleep
As can be understood from Table 1, the brain's frequency, or brainwave, represents the human's mental state that varies instantaneously, and it is discovered that if the brainwave is changed by compulsory brainwave induction, the human's mental state varies. Accordingly, the brainwave inducer is devised to artificially induce the human's mental state.
The method for inducing the brainwave is used for various commercial products, and in many cases, since the sine waves used in this method offend the ears, most products change the reference frequency or add slight noises to the sine waves so that the user can comfortably hear the sine waves.
However, in the method of changing the reference frequency or adding slight noises, the user should be patient in hearing the sound offending the ears for a long while. This is because the user should hear two sine wave sounds having different frequencies for a specific time or longer, in order for the binaural beat effect to last, inducing the brainwave.
Meanwhile, many Internet education companies use the binaural beat effect in order to improve student's concentrations during the study, and to this end, they simply add sine waves to the existing contents and reproduce them, thereby inducing the brainwave. However, even in this service, since the user should hear the sine waves offending the ears for a long time, he/she should annoyingly adjust the volume of both the contents and the sine waves.
Recently, research is being conducted on a method for generating binaural beats using the general voice signal, and the corresponding research generates information on Left (L) and Right (R) channels using a frequency shift block. When the mono voice signal is used in this way, it is possible to generate a natural binaural beat even without using the sine waves.
However, as for the general voice signal, its many parts are composed of silent sounds. Therefore, when the binaural beats are generated using silent periods other than the actual sound periods, the binaural beat effect occurs only in the sound periods without occurring in the silent periods. Thus, the user may feel interruption, and can not be exposed to the binaural beat effect for an extended time, causing a reduction in the brainwave induction effect. Recently, therefore, a brainwave inducer has been proposed, which has resolved the interruption problem using a voice detector.
The brainwave induction method can also be applied to a stereo audio signal, and a description thereof will be made with reference to FIGS. 1 and 2. FIG. 1 is a block diagram illustrating an internal structure of a brainwave induction apparatus according to the prior art, and FIG. 2 is a diagram illustrating two sine waves having different frequencies generated in a conventional brainwave induction apparatus.
Referring to FIGS. 1 and 2, after a mono signal generator 101 generates a mono signal by mixing input stereo audio signals, a frequency shifter 103 shifts a reference frequency 200 in the horizontal frequency direction at regular intervals, generating a shifted frequency 210.
However, in the above simple frequency shifting scheme, the stereoscopic feeling necessary for music listening may completely disappear in the process of mixing the stereo audio signals into a mono signal. In this case, vocal signals which are the signals existing in common in both channels of the stereo audio signals are emphasized, but accompaniments being input to the two channels with different phases and magnitudes are relatively reduced, causing considerable degradation of sound quality. Therefore, the conventional brainwave induction method proposed in FIGS. 1 and 2 cannot be directly applied to the stereo audio signals.